Mercury may be naturally occurring in and around natural gas and crude oil fields. As such, conventional methods have been developed to detect the presence of mercury in connection with oil and gas field operations. In accordance with one such conventional method, fluids contained within gas-rich reservoirs are typically characterized initially with Drill Stem Tests (DSTs) and Wireline Formation Tests (WFTs using tools such as the MDTTM or Modular Dynamic Formation Tester). In gas fields that rely on a relatively small number of wells (i.e., less than about 20 wells), WFTs are run at least once per well with the ability to acquire multiple samples. However, other gas fields require thousands of wells to develop a field and, while testing costs per well may be lower, usually by eliminating DSTs and reducing the number and/or scope of WFTs, overall testing costs for comprehensive field-wide fluid characterization may be higher.
Conventional laboratory testing methods for measuring mercury concentrations in natural gas and crude include exposing gas samples to a gold sorbent to capture mercury compounds from the mixture. The mercury compounds are then thermally desorbed and a total mercury concentration is measured using atomic fluorescence spectroscopy. This method is applicable to both organic and elemental forms of mercury with a detection limit down to about 0.001 μg/m3. In comparison to such laboratory measurement, the measurement of mercury in natural gas in a reservoir is much more difficult, mainly due to the temperature and pressure conditions that are characteristic of reservoir settings and the difficulty in obtaining and transferring a representative sample from the reservoir for testing above ground. Detection limits under these conditions for total mercury are at best on the order of 1 μg/m3 or larger. Due to the changes in conditions, only total mercury measurements are reliable. Specific mercury compounds can be transformed by the process of capturing and moving the sample. Sensor technologies targeted at detecting Hg2+ ions in water solutions at ambient temperatures and pressures are reported in the literature. These technologies primarily exploit the optical properties (i.e., fluorescent assays and colorimetric assays) of activated gold nano-particles. Aside from the focus of these sensors on the detection of mercury ions in water solutions, the practical lower detection limits of such methods is generally not better than 20 ppb, which is consider to be too high to be useful in characterising mercury compounds in natural gas or crude oil.
As such, a need exists for a sensor that is capable of detecting and speciating organic and inorganic mercury compounds in gas, crude, or water to a practical quantification limit of 1 μg/m3 or lower.